This invention relates generally to flywheels used for kinetic energy storage and more specifically to a low mass, variable stress-distribution hub used in such flywheels.
The principle of the flywheel, which has been recognized for a very long time, is that a spinning wheel stores mechanical energy. Until recently, it was thought that employing flywheels to store energy for modern technological applications was out of the question because of the cost and the low efficiency of energy storage as compared to flywheel weight. However, this picture has been radically changed by recent advances in materials technology and in flywheel design.
The amount of energy stored in a flywheel depends upon the mass of the rim and the angular velocity of the wheel. Energy storage varies as the square of the rotational velocity. In theory, the amount of energy that may be stored in a given flywheel may be increased indefinitely with the speed of the flywheel. However, as is well known in the art, there is a limit to the amount of energy that may be stored in a given flywheel which is dependent upon the tensile strength of the material from which the flywheel is constructed and the manner in which the various stresses that are created are distributed in the flywheel. For example, it has been determined that for a given flywheel weight, the best material for storing the most energy consists of a material which is of extremely low density to reduce the stresses in the wheel and which is extremely strong to withstand the stresses that are created. In so far as flywheel design is concerned, it is also known that the mass located towards the rim of the wheel contributes far greater to the energy storage than mass located towards the center of the wheel.
Flywheels have traditionally been made of metal such as high strength steel. However, because of its high density, steel is not suitable for making a flywheel capable of storing large amounts of energy for a given weight flywheel. It has been found that materials comprising a composite of fiber have much more suitable properties for flywheel construction. Such fiber composite materials are much lower in density than steel, while being at least equally strong and far stronger in some cases than the strongest steel alloys. Although advantageous as compared to steel or steel alloy rims, rims comprising fiber composite materials do present certain disadvantages of their own. For example, because fiber composite material rims typically have a lower modulus of elasticity then the remaining portions of the flywheel, the fiber composite rim may expand away from a hub causing dynamic imbalance in the flywheel. The present invention precludes this problem and thus overcomes a major disadvantage of the prior art by a unique geometrical design which permits the higher modulus metal hub to expand during high speed rotation, an amount equivalent to the expansion of the fiber composite. This equalization of expansion permits retention of a firm contact between the inner portion of the fiber composite rim and the outer surface of the hub and thus maintains dynamic stability of the flywheel.
Also traditional in flywheel construction has been the use of mutli-element structures including a centrally located hub which is used to interface the flywheel with the source of rotational input energy as well as with the means of transferring output energy stored in the flywheel. Also included have been the rim portion of the flywheel and inter-connecting devices to secure the rim to the hub. Such multi-piece construction results in a number of disadvantages including the added expense of production and the inferior dynamic stability of the rotating flywheel. In addition, the mass of the connectors and hub add greatly to the inefficiency of energy storage for a given weight flywheel and thus even more, add to the cost of a flywheel for a given amount of energy storage.
The present invention also overcomes the disadvantages derived from multi-element construction by providing a low mass single piece axle and hub upon which a fiber composite material rim may be directly connected without any interconnecting devices.